Sylvia doesn't know she's a girl, or at least doesn't realise her gender isn't set by her clothing. At bedtime she gets very excited about putting on her "big boy" pull-up, believing her pull-up makes her a big boy. From the time she puts her "big boy" pants on until the time we deprive her of them in favour of "big girl" panties, she goes around claiming to be a big boy.
The confusion doesn't end with gender either. The other day she was trying to climb the pantry to get to her Halloween candy. I told her to get down. In response, she said, "No, I monkey!"
Apparently Paul's big sister is actually a big boy monkey.